Structural insights can shape new thinking and contribute paradigm-shifting impact. Telomeres are central to cancer and aging. Questions of structure are central to telomere function where signaling to the DNA repair pathway likely depends on physical changes in the telomere. Previous work from this laboratory led to the discovery of telomere looping (t-loops) and small telomeric DNA circles (t-circles) which have now been found from yeast to humans and likely contributes to telomere maintenance. A new player discovered by others is telomeric RNA bound at the telomere. hnRNPA1 protein binds this RNA tightly and recent work in this laboratory revealed that hnRNPA1 will unwind duplex mammalian telomeric DNA. This research program applies a unique combination of biochemistry and transmission electron microscopy (EM). New EM tagging methods that identify proteins in multiprotein complexes have been developed and will be applied along with new ultra-gentle preparative methods, cryoEM and single particle reconstruction methods, melded with biochemical assays. In AIM I, the core telomere binding proteins (TRF1, TRF2, Pot1, TPP1, hRap1, and Tin2) highly purified and in hand together with co-complexes of these proteins assembled in vivo will be assembled onto model telomere templates, their structure examined, and their ability to remodel telomeric DNA determined. In AIM II, experiments using transgenic mice will further probe role of TRF2, and work on the Rad51 paralogs and the WRN helicase will be conducted to examine the interaction of these repair factors with the telomere complexes on telomeric DNA. AIM III will focus on telomeric RNA and hnRNPA1 in their ability to form a scaffold at the telomere upon which other core telomere binding factors may assemble. The role of hnRNPA1 in facilitating looping and replicative extension of the telomere will be investigated. Aim IV continues a long standing collaboration with the Tomaska group and the discovery of a novel new yeast species with long telomeres and telomere binding protein highly homologous to human TRF1/2. This yeast should provide a better model for human telomere biology. The high productivity of the past funding period provides a strong metric for future success and this is bolstered by strong collaborations. These studies have very high impact since no other laboratory is applying this technology to telomere/repair work and many other laboratories depend on the input from these structural studies. .